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Two new gangliosides were isolated in pure form from beef brain. They were provisionally named ganglioside GSa and G5b. Ganglioside G5a is a monosialoganglioside containing fucose. Its basic neutral glycosphingolipid core is the gangliotetraose ceramide:most likely with b-linkages. Fucose is linked to the 2-position of external galactose. N-acetylneuraminic acid to the 3position of internal galactose. Ganglioside G5b is a mixture of at least two isomeric disialogangliosides containing N-acetylneuraminic acid and N-glycolylneuramink acid. The major isomer has the following structure: NeuNac (2,2 -3) Gal @,I -3) GalNac Cp. 1 -4) (NeuNglrr, 2 + 3) gal (1.1 + 4) Glc (p.1 -)-ceramide. The minor isomer contains N-acetylneuraminic acid and N-glycolylneuraminic acid in an inverted linkage position.

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—The activity of brain membrane‐bound neuraminidase on endogenous and exogenous substrates was comparatively studied in various animals (rat, chicken, rabbit, pig, calf and human). The maximum rate of hydrolysis of endogenous substrates by membrane‐bound neuraminidase (using a crude preparation of the enzyme) was different in the various animals (from 0·05 to 0·73 units, referred to 1 mg protein) and was obtained under similar but not identical optimum conditions (pH from 4·1 to 5·1; requirement or not of Triton X‐100). The maximum degree of hydrolysis of endogenous substates was also different (from 15 to 27 nmol released NeuNAc/mg protein) and was obtained within different incubation periods (from 2 to 18 h). It corresponded (in rabbit, calf, human brain only), or not, to the actual exhaustion of the endogenous substrates. The endogenous substrates were recognized as both gangliosides and sialoglycoproteins. The extent of hydrolysis of sialoglycoproteins varied from 1·5% in rabbit to 15·6% in chicken brain; the hydrolysis of gangliosides (ranging from 14·1% in pig to 53·7% in rabbit brain) reached only in some animals (rabbit, calf, human) the complete transformation of major oligosialogangliosides into the neuraminidase resistant monosialoganglioside GMI. Upon addition of exogenous substrates (sialyl‐lactose, ganglioside GD1a, brain sialopeptides, ovine submaxillary mucin) the actual rate of liberation of total NeuNAc (from both endogenous and exogenous substrates) considerably exceeded, although at a different extent (depending on the animal and on the added substrate used) the rate of hydrolysis of sole endogenous substrates. The possibility of an accurate assay of brain membrane‐bound neuraminidase in a crude enzyme preparation is evaluated and guidelines for the assay procedure suggested.

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